This invention relates to a process of separating, by solvent extraction, metal compounds contained in aqueous solutions. More particularly, it relates to the separating by means of hydroxyquinolines of at least one element, selected from the group consisting of the rare earths and yttrium, from an aqueous solution containing it.
It is well known that the rare earths, that is to say, the lanthanides, which have atomic numbers of between 57 and 71, and yttrium, are difficult to separate from one another due to the similarity of their properties.
It has already been proposed to separate the rare earths and yttrium contained in aqueous solutions by subjecting them to extraction with an organic solution comprising, in particular, either neutral or acid organophosphorus compounds or amines or quaternary ammonium salts, or else carboxylic acids. However, such processes are not always entirely satisfactory since, in certain cases, the solvents used do not have sufficient separation factors or may lead to extensive operating expenses, particularly in case of the regeneration of the acid organophosphorus compounds.
It has also already been proposed, in accordance with an article published in Compte rendu hebdomadaire de l'Academie des Sciences de Paris, Volume 273, of Oct. 27, 1971, Series C, pages 1073 to 1076, selectively to extract europium in bivalent state from the other rare earths having a degree of oxidation of +III by [7-.alpha.(O-carbomethoxyanilino)benzyl]8-hydroxyquinoline or CMAB-oxine in solution in chloroform. In accordance with the authors of said article, the results obtained are different from those expected, since the Eu.sup.3+ ion is extracted slightly before the Eu.sup.2+ ion. However, the separation of Eu.sup.2+ and Eu.sup.3+ should be possible by superimposing on the extraction reaction a reaction of formation of complexes in the aqueous phase with EDTA (ethylene diamine tetraacetic acid).
The +III degree of oxidation is the normal form of the rare earth ions, but some elements have the possibility of being stabilized easily at a higher degree of oxidation (+IV in the case of cerium and praseodymium) or lower degree of oxidation (+II in the case of europium and samarium). The change in the degree of oxidation corresponds, in addition to the variation of the ionic radius of the ion in question, to substantial variations in the chemical properties. It is these differences, which are well known to the man skilled in the art, which it has been attempted to utilize in the process described above for separating europium in bivalent state from the other rare earths in trivalent state. In addition to the apparent lack of success of the results obtained, the solution recommended has the drawback of requiring the use of a complexing agent in addition to the extracting agent.
The values of the extraction constants of certain rare earths with oxine (8-hydroxyquinoline) have already been mentioned in the literature (Pyatnicky I. V. and Gavrilova E. F., Zh. Anal. Khim., Volume 25, page 445, 1970). The values of the extraction constants indicated for neodymium and samarium were determined during the course of different experiments. The value of the extract constants obtained from a given series of experiments is also given in this article for dysprosium, holmium, erbium, thulium, and ytterbium. From these sparse results, which come in most cases from a very inaccurate determination, the man skilled in the art can in no way prejudge the selectivity of oxine for the separation by extraction of at least one rare earth from an aqueous solution containing a mixture of rare earths.
Moreover, it can be noted that oxine has a large number of drawbacks from an industrial standpoint, in particular, too great a solubility in water which results in substantial losses, low capacity of charge, and low solubility in the organic solvents used industrially in liquid-liquid extractions, such as kerosene or Solvesso.
By the present invention it has been discovered that a new class of solvents obviates the said drawbacks and makes it possible to effect the separation from an aqueous solution of at least one element selected from among the group consisting of the rare earths and yttrium under particularly advantageous conditions.
More precisely, the process in accordance with the present invention makes it possible to separate from an aqueous solution at least one element selected from the group consisting of the rare earths and yttrium in the trivalent state, and to do so with excellent selectivity, due to the fact that the separation factors are larger and increase as a whole in the direction of the increasing atomic numbers of the rare earths, that is to say, from lanthanum to lutetium, yttrium occupying an intermediate position.
It is, accordingly, a principal object of the present invention to provide a novel process for separating rare earths and yttrium, one from another, from their aqueous solutions.
Other objects of the invention will be apparent to those skilled in the art from the present description.